The feasibility of using ground water for irrigation in the Dakawa- Wami plains of Tanzania.

Reference to direct sources of water supply is usually with respect to surface water (rivers, lakes etc). Although endewed with a fair distribution of perennial streams, the area under study does not satisfy the technical and economic criteria necessary for their exploitation beyond that required for domestic consumption. Therefore, ruling out the use surface water on grounds of infeasibility, this study investigates the annual basin yield of a potential area in the northern Morogoro Region of Tanzania, which may be justifiably used irrigation in the available erable land. Since the concept of safe yield adopted is that in which this ensuing drawdown due to pumpage would not lead to undesirable consequences such as subsidence or a permanent depletion of the aquifer, a safe approach therefore, is the assumption that the available water for irrigation is the average annual recharge to the area. In evaluating this, the method adopted is by the separation of baseflow from streamflow hydrographs. The estimated average annual recharge of 95.8 x 10º cubic meters through the study area (mainly by underflow), is checked through the use of a groundwater flow model, "FLOW", incorporating the finite element method of analysis. On deduction of the irrigation demand per hectare, the extent of arable land irrigable under the present water supply conditions is 5000 hectares. The question is whether the market demand for the produce (in this case rice) generated at such a limited scale would yield the necessary financial benefits to offset the investment and running costs within a reasonable period of operation. An estimation of the growth development of the scheme in terms of annual costs and revenues, marched through time, gives economic parameters such as the discounted benefit cost ratio, payback period and accounting and internal rates of return. A sensitivity analysis indicates a benefit cost ratio of 1.43 at a minimum attractive rate of return of 6 per cent.